The invention relates to the general field of materials.
More particularly, it relates to estimating the porosity ratio of a material (ratio per unit area or per unit volume), such as for example a composite material. In known manner, the porosity of a material characterizes the pore content of the material, i.e. its content of interstitial voids that may optionally be interconnected.
The invention thus has a preferred but non-limiting application in the field of aviation.
It is nowadays common practice to use primary structures made of composite material in the production of aircraft (e.g. turbojet blades, etc.). Such structures are subjected to strict quality control during which the volume porosity ratio of the composite materials is monitored closely. The presence of porosities in the material can be detrimental to good mechanical strength, so the purpose of the inspection is to make sure that the volume porosity ratio of the material does not exceed a predefined limit value.
In order to determine the porosity ratio of a composite material, it is known to have recourse to a technique of degrading or dissolving the matrix of the material (e.g. chemically by acid attack or by calcination).
In that technique, readings are taken of the weights of a sample of composite material before and after dissolving its matrix. On the basis of knowledge of the densities of the fibers and of the matrix of the material, these weight readings make it possible to calculate easily the volume porosity ratio of the composite material.
Nevertheless, that technique presents a certain number of drawbacks.
Firstly, it depends strongly on the accuracy with which the weights are read and also on the knowledge of the densities of the fibers and of the matrix of the composite material.
Furthermore, that technique is destructive: it relies on totally dissolving the matrix of the composite material. Unfortunately, not only does such dissolution take a long time (several hours), but there also remains doubt as to whether the matrix has been dissolved in full. Furthermore, the presence of foreign particles or ingredients in the material, such as for example inclusions of metal or of glass fiber, have a major effect on calculating the volume porosity ratio.
Finally, that technique is difficult to apply industrially to materials made of metal or to composite materials having a matrix that is made of ceramic or of metal.
The document by Y. Ledru et al. entitled “Quantification 2-D et 3-D de la porosité par analyse d'images dans les matériaux composites stratifiés aéronautiques” [2D and 3D quantification of porosity by analyzing images of stratified aviation composite materials], JNC 16, Toulouse, 2009, proposes a technique of estimating the porosity ratio of a composite material in non-destructive manner on the basis of analyzing images that are gray-scale coded.
More particularly, it proposes isolating in said images pixels that correspond to porosities and pixels that correspond to matter. The volume porosity ratio of the composite material is then deduced from the number of pixels corresponding to porosities as isolated in this way.
Nevertheless, that technique relies on an operator setting a gray-scale value threshold for distinguishing pixels. Operator action makes the analysis undertaken in that document subjective and therefore difficult to verify or to perform reproducibly.